Device for reducing thermal stress on connection points

ABSTRACT

The present invention relates to a device adapted in order to decrease stress on connection points between a heat generating source and a substrate. The device  13  comprises a larger heat-dissipating part  7 , and at least one smaller heat-dissipating part  6 . The larger part  7  is arranged with at least one cavity  8  for housing the at least one smaller part  6 . The at least one smaller part  6  is adapted to be attached to at least one heat-generating source  2 , and at the same time more mobile in the cavity  8  and/or less affected by changes in temperature than the larger part.

CROSS REFERENCE TO RELATED APPLICATION

This application is a 35 U.S.C. §371 national stage application of PCTInternational Application No. PCT/SE2007/051016, filed on 17 Dec. 2007,the disclosure and content of which is incorporated by reference hereinin its entirety. The above-referenced PCT International Application waspublished in the English language as International Publication No. WO2009/078767 A1 on 25 Jun. 2009.

TECHNICAL FIELD

The present invention relates to a device, a radio base station (RBS)and an apparatus for decreasing stress on connection points betweenheat-generating sources and a substrate. The present invention moreparticularly relates to reduction of stress induced by thermalexpansion.

BACKGROUND

Electronic devices in general and power amplifiers in particular, thatare common in many electronic devices such as radio base stations,operate with limited efficiency. Therefore a significant fraction oftheir power consumption turns into heat. It follows that the thermaldesign of these devices is important. Such devices are typicallyair-cooled and their mechanics serve as thermal interface between thehot devices and the air. The heat-generating source, such as a poweramplifier transistor or microprocessor, typically attains its thermalcontact with the heat-dissipating device by bolting or soldering. Theheat-dissipating device can be a cooling flange or heat sink with orwithout active cooling such as a fan. Heat sinks function by efficientlytransferring thermal energy (“heat”) from an object at high temperatureto a second object at a lower temperature with a much greater heatcapacity. This rapid transfer of thermal energy quickly brings the firstobject into thermal equilibrium with the second, lowering thetemperature of the first object, fulfilling the heat sink's role as acooling device. Efficient function of a heat sink relies on rapidtransfer of thermal energy from the first object to the heat sink, whichis designed to efficiently dissipate thermal energy to the surroundingair.

Traditionally, the heat-generating device is soldered to a substratesuch as a printed circuit board (PCB) as well. In operation, heatgeneration will cause both the PCB and the cooler to expand. Since theynormally are made of different materials, the magnitude of expansionwill be determined by their respective coefficients of thermalexpansion. A flange, heat sink, is typically made of metal such asaluminum or copper and will therefore expand more than a PCB which istypically made of plastic laminate or other similar materials. A flangemight alternatively be made of an alloy. This difference will impart aforce on the connection between the PCB and the cooling flange.Ultimately it will also impart a force on the solder joints between theheat-dissipating device and the PCB that it sits on, since transistorsand other heat-generating devices are attached to both the PCB and thecooling flange. As the component work load varies during operation, thetemperature will cycle causing variation in the mentioned force, whichwill lead to stress on the connection points.

Experience has shown that this stress will cause the solder joints tofail prematurely, requiring the soldered component or even the completedevice to be replaced. This is a costly process, in addition to theinterruption of operation. The stress effect is even more pronounced ifthere is more than one component with significant heat generation in thedevice.

Patent document U.S. Pat. No. 6,128,190 A describes a transistor clampprovided to obviate solder joints and stress imparted hereon. Howevernot all components are suited for this solution. Due to their design orfor other reasons, some components require soldering or similar fixingmethods.

SUMMARY

One object of the present invention is to provide a device in whichthermal stress on connection points is reduced and thereby serviceintervals are increased compared to above mentioned prior art.

Another object of the present invention is to provide an apparatus withreduced stress on connection points, which is simple to manufacturewithout significantly impacting design and production.

Some advantages of the present invention are highly reduced stress levelon connection points such as solder joints, increased mean time betweenfailures of an apparatus, higher reliability, less down-time etc. Aspin-off effect arising from the present invention is the possibility touse a heat-dissipating device for cooling a variety of heat-generatingsources.

An embodiment of the present invention provides a device comprising alarger heat-dissipating part, and at least one smaller heat-dissipatingpart. The larger part is arranged with at least one cavity for housingthe at least one smaller part. The at least one smaller part is adaptedto be attached to at least one heat-generating source.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a device according to prior art.

FIG. 2 is a top view illustrating embodiments of the present invention.

FIGS. 3 a and 3 b illustrate side views of different embodiments of thepresent invention.

FIGS. 4 a-4 d illustrate examples of different shapes of smallerheat-dissipating parts, according to embodiments of the presentinvention.

FIG. 5 illustrates a side view according to a further embodiment of thepresent invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a prior art Printed Circuit Board 1(PCB), substrate, comprising cavities 4 and one or more heat-generatingsources 2 in the cavities. The heat-generating sources are typicallysoldered to the substrate by the aid of connecting means 3, which aretypically prefixed to the heat-generating sources 2. A heat-dissipatingpart 5, flange or heat sink, is used to cool the heat-generating sourceswhen active e.g. when for example the transistor is in use. The PCB istypically made of plastic laminate and the heat-dissipating parts,coolers, are made of aluminum or other thermally conductive material. Aphysical mechanism between failing solder joints is the difference inthermal expansion between plastic laminate and aluminum and the factthat heat-generating sources, like transistors, are affected by both thePCB and the cooler.

FIG. 2 is a top view illustrating a device according to an embodiment ofthe present invention. The device 13 comprises a larger heat-dissipatingpart 7 (dashed line), and a smaller heat-dissipating part 6 (dashedline). The larger part 7 is arranged with at least one cavity 8 (dashedline) for housing the smaller part. The smaller part 6 is adapted to beattached to a heat-generating source 2, e.g. a transistor. The presentinvention is not limited to PCB cavities 4 housing one heat-generatingsource 2 or to the fact that each small heat-dissipating part 6 isattached to one heat-generating source 2. Different combinations ofnumber of sources 2, per cavity 4, per part 6 exist and are used eventhough not shown in the figure. Also, the present invention is notlimited to one cavity for housing one or more smaller parts 6. Anycombination and/or any number of smaller parts 6 are possible.Typically, a PCB 1 comprises several transistors 2, microprocessorsand/or any other electrical components in different locations anddirections of the PCB. Therefore, there is a need to divide theheat-dissipating device 5 into several smaller parts 6 each interfacing,covering, one or more heat-generating sources 2. By dividing theheat-dissipating device 5 into a smaller part 6 interfacing theheat-generating source 2, the stress in solder joints is experimentallyproven to be decreased. Thus, the smaller part is less affected bychanges in temperature variations than the larger part 7 due to itssize. The heat-dissipating parts together are this way adapted to forman effective heat sink.

Further on if the device 13 comprises at least one cavity 8 that isbigger than the size of the smaller heat-dissipating part 6 that isintended to be housed in that cavity 8, then the smaller part 6 is moremobile in its cavity 8. This decreases the stress even more since thereis some mobility relative to the underlying surface. Additionally, it ispossible that each cavity 8 can be adapted to house one or more smallerparts 6.

FIGS. 3 a and 3 b illustrate side views of different embodiments of thepresent invention. Two different shapes of smaller parts (6 a; 6 b) aredescribed in FIG. 3 a and FIG. 3 b. Since the device 13 typicallycomprises soldered connection points, between the connecting means 3 andthe substrate 1, some of the stress on the connection points can bereduced by having lead shaped connecting means 3 b (prior art). Usinglead shaped connecting means 3 b is adapted mainly to reduce stress inone direction, which is sideways looking at the heat-generating source.However some of the stress is reduced in other directions as well basedon the flexibility of the lead shaped connecting means 3 b. Typically,the heat-generating source is manufactured comprising connecting means(3 a; 3 b) or the connecting means (3 a; 3 b) are fixed to the source 2before delivery to customers.

In another embodiment of the present invention the device 13 furthercomprises a thermally conductive layer 9, such as aluminum or copper,between the at least one heat-generating source 2 and the at least onesmaller heat-dissipating part 6. This layer is needed to compensate forthickness variations between different heat-generating sources, and canalso be used as part of a fixing means needed to fix the source 2 to thesmaller part 6. In yet another embodiment of the present invention,there is another thermally conductive layer interposed between thesmaller 6 and the larger 7 heat-dissipating parts, such as thermal paste(not shown in figure). One purpose of using the other thermallyconductive layer is to fill up gaps between the heat source 2 and thesmaller part 6.

According to yet another embodiment of the present invention the device13 comprises at least one smaller part 6 and one larger part 7 whereinat least one smaller part 6 of the device is of the same material as thelarger part 7. However this is a non limiting embodiment. For examplecopper can be used for one or more smaller parts 6 while aluminum isused for the larger part 7. Also other materials or alloys are possibleto use, like AlSiC (Aluminum Silicon Carbide) as an example. Thus, it ispossible according to the present invention to manufactureheat-dissipating devices with one type of larger part 7 and one or moretypes of smaller parts 6 depending on the needs and current materialcost.

FIGS. 4 a-4 d illustrate examples of different shapes of smallerheat-dissipating parts, according to embodiments of the presentinvention. According to these embodiments the device 13 comprises atleast one smaller part 6 which is being adapted to the shape of the atleast one heat-generating source 2. This is achieved by one or more ofthe following: a hollowed-out section of the smaller part 6, FIG. 4 aand FIG. 4 b; a protrusion of the smaller part 6 into the cavity 4, FIG.4 c and FIG. 4 d; an extra layer 9, as mentioned above, of similarmaterial as the small part 6 between the smaller part 6 and theheat-generating source 2. FIGS. 4 a-4 d are non-limiting examples givento explain some of most typical implementations of the presentinvention. However other shapes of the cavity or the protrusion arepossible even though not shown in the figures. Also, the extra layer 9between the smaller part 6 and the heat-generating source 2 hasproperties such as shape, thickness and material that can be varieddepending on needs and requirements. In some embodiments, the extralayer 9 is not present at all.

FIG. 5 illustrates a block scheme according to another embodiment of thepresent invention wherein the device 13 further comprises fixing means(9; 10; 11; 12) for attaching the at least one heat-generating source tothe at least one smaller part 6 of the device 13. Such fixing means arefor example a yoke or a clamp 10 that is bolted or screwed 12 to thesmaller part 6 in such a way that pressure is applied to theheat-generating source, assuring reliable contact with the cooling parts(6; 9). A fill-in part 11 is used to separate the heat generating source2 from the clamp 10 so that the clamp does not effect on the technicalfunctionality of the heat generating source 2. Another purpose with thefill-in part 11 is to fill in gaps caused by that the heat-generatingsource 2 is of thinner size than calculated and/or designed for. Also,even though soldering is not shown in the figure it is still possible tosolder the heat source 2 to the smaller part 6 directly and/or to thesubstrate if required or requested. In FIG. 5 only a non limitingexample is illustrated for describing this embodiment of the presentinvention in a simple manner. However other shapes and kinds of fixingmeans (9; 10; 11; 12) are possible to use.

An important benefit achieved by the present invention according to theexample mentioned above is that the heat-generating sources are directlybolted or soldered to the smaller heat-dissipating part 6 and are freeto move back, forth and sideways in relation to the largerheat-dissipating part 7. The heat source 2 is in this way adapted tofollow the substrate, PCB, and “ignore” the larger heat-dissipating part7 when the larger part 7 and the substrate 1 move relative to each otherdue to differences in thermal expansion. Pressure is applied on thesmaller part 6 by aid of substrate design in such a way that a topsurface of the smaller part is typically in same level or higher than atop surface of the larger part 7. The top surface of the larger part 7is the surface facing the substrate.

It will be understood by those skilled in the art that variousmodifications and changes may be made to the present invention withoutdeparture from the scope thereof, which is defined by the appendedclaims.

1. A device comprising: a larger heat-dissipating part; and at least onesmaller heat-dissipating part, wherein the larger part comprises atleast one cavity that houses the at least one smaller part, and whereinthe at least one smaller part is attached to at least oneheat-generating source, wherein the at least one smaller part is adaptedto the shape of the at least one heat-generating source by comprisingone or more of the following: a hollowed-out section of the smallerpart; a protrusion of the at least one smaller part extends into thecavity; an extra layer of similar material as the smaller part isbetween the at least one smaller part and the heat-generating source. 2.The device of claim 1, further wherein the at least one cavity biggerthan the smaller heat-dissipating part which is intended to be housed inthat cavity.
 3. The device according to claim 1, further comprisingfixing means for attaching the at least one heat-generating source tothe at least one smaller part of the device.
 4. The device according toclaim 1, further comprising a thermally conductive layer, whichcomprises a thermal paste, between the at least one heat-generatingsource and the at least one smaller heat-dissipating part.
 5. The deviceaccording to claim 1, wherein the at least one heat-generating source islocated in at least one cavity of a substrate, and wherein the substrateis a printed circuit board (PCB).
 6. The device according to claim 5,wherein the at least one heat-generating source is soldered to thesubstrate at one or more connection points by one or more connectingmeans.
 7. The device according to claim 6, wherein the one or moreconnecting means are lead shaped adapted to decrease stress on the oneor more connection points in a certain direction, the stress caused bydifferent thermal expansion in the device.
 8. The device according toclaim 1, wherein the at least one smaller part of the device is of thesame material as the larger part.
 9. The device according to claim 1,wherein the at least one smaller part of the heat-dissipating device isof different material than the larger part.
 10. The device according toany preceding claim, wherein the at least one heat-generating sourcecomprises a transistor, a microprocessor, and/or another integratedcircuit chip.
 11. The device according to claim 1, wherein the largerheat-dissipating part and the at least one smaller heat-dissipating partare adapted to form a heat sink.
 12. The device according to claim 1,wherein the at least one smaller part is made of different shapes.
 13. Aradio base station comprising the device of claim 1 connected to receiveand dissipate heat from an electronic component that provides anoperational function of the radio base station.
 14. An apparatuscomprising: a substrate; at least one heat-generating source; at leastone heat dissipating part that decreases stress, caused by a change oftemperature and different coefficients of thermal expansion of the atleast one heat-dissipating part and the substrate, on one or moreconnection points between the at least one heat-generating source andthe substrate, wherein each of the at least one heat-generating sourceis located in a cavity of the substrate, and each of the of the at leastone heat-dissipating parts is attached to both the substrate and to theat least one heat-generating source, and wherein each of the at leastone heat-dissipating parts comprising a smaller part with an area thatinterfaces the at least one heat-generating source, and a larger partarranged with a cavity for housing the smaller part, wherein the smallerpart is more mobile in the cavity and/or less stress affected by changesin temperature than the larger part, wherein the at least one smallerpart is adapted to the shape of the at least one heat-generating sourceby comprising one or more of the following: a hollowed-out section ofthe smaller part; a protrusion of the at least one smaller part extendsinto the cavity; an extra layer of similar material as the smaller partis between the at least one smaller part and the heat-generating source.15. The device according to claim 1, wherein the at least one smallerpart is adapted to the shape of the at least one heat-generating sourceby comprising a protrusion that extends into the cavity.
 16. The deviceaccording to claim 1, wherein the at least one smaller part is adaptedto the shape of the at least one heat-generating source by comprising ahollowed-out section that conforms to a shape of the at least oneheat-generating source.
 17. The device according to claim 1, wherein theat least one smaller part is adapted to the shape of the at least oneheat-generating source by comprising an extra layer of similar materialas the smaller part between the at least one smaller part and theheat-generating source.